1. Field of the Invention
This invention relates to methods and apparatus for controlling combustion to minimize nitrogen oxide emission and, more particularly, to the use of flame spectroscopy to control boiler burner combustion in a manner that reduces nitrogen oxide emissions without increased risk of flame-out.
2. Description of the Related Art
A boiler burner is a combustor for heating water to generate steam to drive a system such as, for example, a steam turbine. Fuel for a boiler burner can include materials such as coal, oil, natural gas or a combination including these materials. As with all forms of combustion, boiler burners emit an exhaust stream that contains various combustion products. While some of these combustion products, such as water vapor, are essentially harmless to the environment, others may not be and, for that reason, are classified as pollutants. Accordingly, a major effort is presently underway to reduce pollutant emissions, especially emissions of various forms of nitrogen oxide, collectively referred to as "NO.sub.x ".
For a steam turbine, NO.sub.x emissions increase significantly as the boiler burner combustion temperature rises. Operating a boiler burner in a so-called "lean-burn" condition, which involves use of a lean mixture of fuel and combustion air (i.e., a relatively low fuel-to-air ratio), reduces the combustion temperature to a level that significantly reduces NO.sub.x emissions. However, if the mixture is too lean, the boiler burner exhibits operational instabilities which may increase to a point at which the combustion flame is extinguished, i.e. a "flame-out" occurs and the boiler burner ceases generating power.
To assure that a flame-out will not occur in an engine, the engine is frequently adjusted to operate with a "rich" fuel/air mixture, i.e., with a relatively high fuel-to-air ratio. While this results in stable engine operation, it also produces high NO.sub.x emission levels. While heretofore such high NO.sub.x emission levels have been tolerated as a cost of safe operation, environmental concerns have heightened to the point at which these emission levels need to be significantly reduced but with no ensuing diminution in operational safety.
Traditionally, most boiler burners for steam turbines rely on using a fuel-to-air mixture that is preset during manufacture and testing to conform with an expected operating condition for the burner, e.g. a mixture that will establish a "rich" condition. Apart from a control for regulating fuel flow, boiler burners generally employ no other adjustments that can be used to dynamically change the burner's operation, let alone change the fuel/air mixture to reduce NO.sub.x emissions.
Currently, there is no known closed-loop feedback technique for controlling a boiler burner to operate in a lean-burn condition. This is due to both the paucity of usable boiler burner adjustments, as noted above, as well as various difficulties associated with accurately detecting the level of NO.sub.x emissions produced and abating these emissions in real-time by adjusting the fuel-to-air ratio of the burners.
One technique involves controlling the fuel-to-air ratio of multiple flames based on measurements of two single spectral lines in each flame: an infrared carbon dioxide (CO.sub.2) line at 4.41 .mu.m (micrometers) and an ultraviolet hydroxyl (OH) line at 300 nm (nanometers), respectively. An intensity ratio based on the measured values of these two lines for each flame is determined and then used to separately control the fuel/air mixture of that flame in order to achieve near stoichiometric combustion, which advantageously occurs at a fuel-to-air ratio that reduces the amount of NO.sub.x that heretofore has generally been emitted. See, e.g., F. Fraim, "Research into a Spectral Flame Analyzer Phase 1--Final Report for the Period Apr. 21, 1983-Jun. 30, 1985", Work Performed under United States Department of Energy Contract DE-AC07-831 D12463, Jun. 1, 1985. Specifically, while suitable detectors, such as photodiodes, exist that can readily sense infrared radiation, the high temperature of certain engines causes various engine components to emit intense amounts of radiation over the entire infrared spectrum. In addition, the boiler flame generally produces a large amount of unburned "slag" particles which themselves function as infrared black body radiators in contrast to a "clean" natural gas flame which does not produce such particles. The resulting background level of infrared radiation can be sufficiently high to completely, or almost completely, mask the radiation associated with the CO.sub.2 spectral line as well as radiation associated with other desired spectral components in the infrared spectrum. Consequently, any technique predicated on measuring radiation in the infrared spectrum produced by the flame would likely be impaired by the radiant energy produced by the hot combustor compartment itself and thus provide highly erroneous measurements.
Thus, a need exists for a technique that can substantially reduce NO.sub.x emissions by operating the boiler burners in a "lean-burn" condition. Such a technique should not rely on detection of infrared emissions and should provide closed-loop feedback control to assure stable burner flames. In addition, the technique should be readily amenable to inclusion in existing boiler burner systems, on a retrofit basis, as well as to inclusion in new boiler burners during their manufacture.